Endocrine Abstracts

Introduction: A pituitary adenoma is directly assumed to originate from anterior pituitary. It is expected that pathologies of posterior hypophysis cause diabates insipitus (DI). However recently posterior pituitary tumors (PPTs) are also declared which are not associated with DI in contrast to what is expected. Rather they have a clinical and radiologic presentation akin to anterior pituitary adenomas. Differentiation of two conditions is mainly based on pathologic immunohystochemical staining of posterior pituitary tumors with Thyroid Transcription Factor-1 (TTF-1), which is universally positive in posterior pituitary tumors but negative in anterior ones. PPTs are believed to originate from pituicytes, which are specialized glia of the posterior pituitary. Incidences of these rare tumors are as low as less than 0.5% of sellar tumors (1). Transsphenoidal surgery (TSS) is the treatment of choice for symptomatic PPT.

Case report: A 44-year-old man presented with headache. Magnetic resonance imaging (MRI) indicated a 22x12.7x13 mm pituitary lesion. Anterior pituitary hormones at presentation were within normal levels. Patient underwent TSS. Patologic evaluation showed a nullcell adenoma with Ki-67 (MIB-1) index of % 5-6. A residual 16x15 mm tumor was detected in the pituitary MRI 3 months after the operation. The patient underwent TSS again at the postoperative 11th month. GH, ACTH, TSH, LH, FSH, pit-1, SF-1 staining was negative in patological examination. TTF-1 was positive. Synaptophysin and S-100 protein were weakly positive. GFAP, EMA and chromogranin were negative. Ki-67 (MIB-1) index was % 15. Due to TTF-1 positivity, PPTs and metastatic neuroendocrine tumors were included in the differential diagnosis. Neuroendocrine tumor metastasis was ruled out as chromogranin, insulinoma-associated protein 1 and cytokeratins were negative. FDG-PET also did not show a primary/metastatic malignancy. On follow-up there was no recurrence or residual tumor and no hormonal deficiency.

Discussion: PPTs generally arise in the region of the sella and present similarly to nonfunctional pituitary adenomas, making it difficult to distinguish them from other common pituitary lesions based on hormonal status and imaging alone. TTF-1 positivity is crucial to diagnosis. Because of the high rate of recurrence, TTF-1 positive tumors should be followed up at close intervals. (1) Shibuya M. Welcoming the new WHO classification of pituitary tumors 2017: revolution in TTF-1-positive posterior pituitary tumors. Brain Tumor Pathol. 2018 Apr;35(2): 62-70.